Update unit tests: add BillingService/CollaborationGateway mocks and adjust app greeting

[Adityavanjre]

Motivation

• Ensure unit tests reflect new service dependencies and updated application status messaging.
• Provide minimal mocks for newly introduced services so tests can instantiate service providers without touching implementation details.

Description

• Added a mock BillingService provider with getPlanByTenant and checkQuota stubs to ledger.service.spec.ts so LedgerService tests satisfy new constructor dependencies.
• Updated the AppController spec expectation string in app.controller.spec.ts to Nexus ERP API Gateway is Operational to match the current status message.
• Added a mock CollaborationGateway with broadcastCommentAdded and broadcastCommentDeleted to audit-collab-001.spec.ts and registered it in the testing module to satisfy CollaborationService dependencies, and added a debug console.log in the delete comment test to emit the actual error message on failure.

Testing

• Ran the unit test suite for the modified specs using Jest (yarn test), focusing on ledger.service.spec.ts, app.controller.spec.ts, and audit-collab-001.spec.ts, and the updated tests completed successfully.
• Verified that the double-entry integrity test still rejects imbalanced journal entries by asserting a BadRequestException is thrown.

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